Image sensors such as CMOS (Complementary Metal-Oxide-Semiconductor) image sensors and CCD (Charge Coupled Device) image sensors, with the improvement in their characteristics, have been widely used in applications such as digital cameras, portable telephones with cameras, and scanners.
Further improvement of characteristics has been desired for the image sensors and one of them is to widen the dynamic range. The dynamic range of the conventionally used image sensors is staying, for example, within a range of 3 to 4 digits (60 to 80 dB) and it has been desired to realize a high quality image sensor having a dynamic range of 5 to 6 digits (100 to 120 dB) or more comparable to the naked eye or a silver-halide film.
As a technique for improving the foregoing image sensor quality characteristics, for example, Non-Patent Document 1 discloses a technique in which, in order to increase the sensitivity and the S/N ratio, a noise signal generated by a floating diffusion adjacent to a photodiode of each pixel and a signal obtained by adding an optical signal to the noise signal are respectively read and a difference therebetween is derived, thereby suppressing noise.
However, even by this method, the dynamic range is about 80 dB or less, and it has been desired to achieve a wider dynamic range.
For example, Patent Document 1 discloses a technique in which, as shown in FIG. 19, a floating diffusion of a small capacitance C1 on the high-sensitivity low-illumination side and a floating diffusion of a large capacitance C2 on the low-sensitivity high-illumination side are coupled to a photodiode PD to out a low-illumination side output OUT1 and a high-illumination side output OUT2, respectively, thereby widening the dynamic range.
Further, Patent Document 2 discloses a technique in which, as shown in FIG. 20, a capacitance CS of a floating diffusion FD is variable, thereby widening the dynamic range. In addition, it discloses a technique in which imaging with a short exposure time adapted to high illumination and imaging with a long exposure time adapted to low illumination are performed, thus widening the dynamic range by division into two or more mutually different exposure times.
Further, Patent Document 3 or Non-Patent Document 2 discloses a technique in which, as shown in FIG. 21, a transistor switch T is provided between a photodiode PD and a capacitance C and the switch T is turned on in a first exposure period to store photosignal charges in both the photodiode PD and the capacitance C and is turned off in a second exposure period to store photocharges in the photodiode PD in addition to the stored charge, thereby widening the dynamic range. In this example, when there occurs light irradiation exceeding saturation, the excess charges are drained through a reset transistor R.
Further, Patent Document 4 discloses a technique in which, as shown in FIG. 22, a capacitance C larger than a conventional one is used as a photodiode PD, thereby enabling adaptation to high-illumination imaging.
Further, Non-Patent Document 3 discloses a technique in which, as shown in FIG. 23, a photocurrent signal from a photodiode PD is stored and output while logarithmically converting it using a logarithmic conversion circuit formed by combining MOS transistors, thereby widening the dynamic range.    Patent Document 1: Japanese Unexamined Patent Application Publication (JP-A) No. 2003-134396    Patent Document 2: Japanese Unexamined Patent Application Publication (JP-A) No. 2000-165754    Patent Document 3: Japanese Unexamined Patent Application Publication (JP-A) No. 2002-77737    Patent Document 4: Japanese Unexamined Patent Application Publication (JP-A) No. Hei 5-90556    Non-Patent Document 1: S. Inoue et al., IEEE Workshop on CCDs and Advanced Image Sensor 2001, pp. 16-19    Non-Patent Document 2: Y. Muramatsu et al., IEEE Journal of Sold-state Circuits, Vol. 38, No. 1, 2003    Non-Patent Document 3: The Journal of the Institute of Image Information and Television Engineers, Vol. 57, 2003